Seen a post elsewhere on people debating this and the conclusion was put at under 20g, but then alot of people on the site that had tanks 20+ and started to say "But we're nano tanks too!" so they bumped it into the under 30 catagory. Then people started complaining b/c they had 30-40g tanks and thought they were nano's. You see where this is going.
IMO anything under 25g is considered a nano, or anything that your doing a 2-4g water change. If your not worried when you go away for a weekend b/c someone is taking care of your tank and your worried they will add 1-5 ml too much of bionic and your tanks gnna explode its not a nano :scratchch. A true nano is a 15 gallon or less. Or anything that your having a hard time filling up your hydrometer to take a salinity reading :biggrin: